Facies characteristics of the Lefika la Noka tufa, Cradle of Humankind, South Africa

The Lefika la Noka (LLN) tufa is the only documented tufa deposit from the hominin fossil-rich Cradle of Humankind (Cradle) in South Africa. It is situated north of the Malapa and Gladysvale fossil sites, within the Grootvlei catchment area of the Cradle. LLN tufa presents an excellent opportunity for paleoclimate reconstruction in the Cradle, particularly for the late Pleistocene – Holocene period, when Homo naledi and archaic Homo sapiens emerge in the Southern African fossil record. To reconstruct the paleoclimate of the Cradle using the tufa deposit, it is necessary to first understand the sedimentological characteristics of the tufa. Therefore, this study aims to describe the LLN tufa facies, depositional environments, and stratigraphy by using field and petrographic techniques. The tufa has a maximum thickness of about 12.5 m. Seven distinct tufa facies were identified from five drilled cores and seven outcrops. These are phytoherm framestones of calcified vertical stems and bryophytes, phytoherm boundstones of stromatolites and thrombolite-like tufa, oncoidal tufa, micritic tufa, and lithoclastic tufa.  These tufa facies were deposited in a fluvial environment, with phytoherm framestones and boundstones precipitated in situ in high-energy environments, such as barrages, cascades, and waterfalls. The oncoids formed within turbulent pools. The micritic tufa precipitated slowly in the calm water pools, whereas the lithoclast tufa accumulated along the river channel during periods of flooding. The lateral and vertical facies distribution indicates the role of hydrodynamics and CO2 degassing during deposition.